A radio communication system built in a vehicle is a system in which an antenna to execute radio communication with a portable terminal device is arranged inside the vehicle interior, such as a vehicle electronic key system. The vehicle electronic key system is a system in which locking and unlocking a door of the vehicle is executed by radio communication using a portable electronic key without using a mechanical key, and generally the system is referred to as smart entry system. The electronic key is a portable terminal device including an authentication function, namely, a kind of key fob. The vehicle electronic key system detects whether the electronic key exists inside the vehicle interior by using a radio wave of a low frequency zone having a frequency of 125 kHz.
A reason for adopting the low frequency zone for the vehicle electronic key system is as follows. Generally, a radio wave is known to have a characteristic of being reflected by a mesh made of conductors (mesh) having an opening size which is 1/10 or less of a wavelength of the radio wave. The vehicle electronic key system utilizes this characteristic. More specifically, the wavelength of the radio wave of the low frequency zone is 2000 m or more. The radio wave of the low frequency zone is reflected by the mesh made of the conductors having a size of 2-meter squares (height 2 m×width 2 m) or less.
A standard vehicle has a window having a size smaller than 2-meter squares. The window can be regarded as the opening of 2-meter squares, namely, the mesh made of the conductors. The radio wave of the low frequency zone transmitted from an antenna arranged inside the vehicle interior does not leak outside the vehicle from the window having the size of about 1-meter to 2-meter squares. Thus, in the case where the radio wave of the low frequency zone is transmitted from the vehicle interior, the vehicle can be regarded as a metallic box (made of the conductors). The radio wave of the low frequency zone repeats being reflected inside the vehicle interior.
The vehicle electronic key system utilizing this characteristic transmits the radio wave of the low frequency zone from the antenna arranged inside the vehicle interior and detects a portable electronic key in the case of locking the door with a lock switch while the portable electronic key is left inside the vehicle interior. In the case where any response is received from the portable electronic key, existence of the portable electronic key inside the vehicle interior can be detected. Therefore, the door is never locked with the portable electronic key left inside the vehicle interior.
Considering portability for a driver, preferably, the portable electronic key is downsized as compact as possible. However, there is a limit to downsize the antenna included in the portable electronic key because the low frequency zone is adopted. Therefore, the inventor of the present invention focuses on a fact that various kinds of portable terminal devices including cell phones and portable information terminal devices (smartphones) are widely used in recent years. In the case where the portable terminal device can be used instead of the portable electronic key, it would be extremely convenient for a vehicle driver. For that, it is only necessary to incorporate the function of the portable electronic key only into the portable terminal device.
However, additionally making a space for mounting the antenna for the low frequency zone in a general portable terminal device is disadvantageous to improve versatility and merchantability of the portable terminal device and reduce the cost. Here, there are many cases in which communication systems based on the standards of Bluetooth (registered trademark) or Wi-Fi (registered trademark) is adopted in a general portable terminal device as standard for short-distance communication other than communication with a base station of a public network. Therefore, from the viewpoints of the cost and versatility, preferably, the communication based on the standards of Bluetooth (registered trademark) or Wi-Fi (registered trademark) is also adopted in the communication system of the portable terminal device including the function of portable electronic key.
However, according to the standards of Bluetooth (registered trademark) or Wi-Fi (registered trademark), a radio wave of a high-frequency zone having a frequency 2.4 GHz is used. A wavelength of the radio wave having the frequency 2.4 GHz is about 12.5 cm, which is extremely short.
According to the above-described relation between the wavelength of the radio wave and reflection of the radio wave, the wavelength is short in the case of adopting the frequency 2.4 GHz. Most of the radio waves are not reflected by the window of the vehicle and directly leak outside the vehicle from the window. In such a state, existence of the portable terminal device inside the vehicle interior cannot be detected even though the radio wave is transmitted from the antenna arranged inside the vehicle interior while the portable terminal device is left inside vehicle.
In recent years, development is made on a vehicle radio communication system in which control signal communication is executed over the radio among a plurality of devices mounted on the vehicle. This kind of vehicle radio communication system is known by Patent Literature 1.
The vehicle radio communication system known by Patent Literature 1 includes: one base station provided with a first antenna and a leaky coaxial cable (LCX); and a plurality of terminal stations configured to communicate with the base station over the radio.
The leaky coaxial cable is installed in an area inside the vehicle where a signal from the first antenna does not directly reach, and functions as a second antenna. A part of terminal stations among the plurality of terminal stations, positioned in the area where the signal from the first antenna cannot directly reach, receives a signal from the leaky coaxial cable.
However, the radio wave of the high-frequency zone transmitted from the leaky coaxial cable leaks outside the vehicle from the window only by simply arranging the leaky coaxial cable in the area where the signal from the first antenna cannot directly reach inside the vehicle. Since the radio wave of the high-frequency zone leaks outside the vehicle from the window even though such a leaky coaxial cable is adopted, for example, as an antenna of the above-described vehicle electronic key system, existence of the portable terminal device inside the vehicle interior cannot be detected.